Secondary flows are important in understanding the performance of a turbocharger. A primary flow is typically very similar to what would be predicted using the basic principles of fluid dynamics. A secondary flow is typically a flow not in the primary flow. Secondary flows move the fluid in a direction not in primary flow direction which, reduces the fluid energy and increase the losses. Nevertheless, in real world situations there are regions in the flow field where the flow is significantly different in both speed and direction to what is predicted using simple analytical techniques. The flow in these regions is the secondary flow. These regions are usually in the vicinity of the boundary of the fluid adjacent to solid surfaces where viscous forces are at work and near areas that have pressure gradients not in the primary flow direction. For example, a secondary flow could flow in a blade to blade direction for a compressor wheel or a turbine wheel.
Many types of secondary flows occur, including tip clearance flow (e.g., tip leakage), and flows at off-design performance (e.g., flow separation). Such secondary flows cause both an overall loss of flow and a loss of fluid kinetic energy. To improve the efficiency of a turbocharger wheel, e.g., a turbine wheel, secondary flow loss and secondary kinetic energy loss may be minimized. In other words, the wheel may be configured for secondary flow control. For example, the wheel may be manufactured for extra-small tip clearances to limit tip leakage (albeit at additional manufacturing expense).
Turbochargers for vehicular internal combustion engines typically have small turbines. As a result, the blade tip clearances may be relatively significant. Thus, these turbines may be particularly susceptible to secondary flow losses.
Accordingly, there has existed a need for a turbocharger wheel having features characterized in that they provide secondary flow control. Preferred embodiments of the present invention satisfy these and other needs, and provide further related advantages.